Japanese Patent No. 2836789 discloses a linear solenoid valve which regulates the oil pressure of a hydraulic device. This prior art employs a three-way linear solenoid valve in a hydraulic circuit which controls the line pressure of an automatic transmission. As shown in FIG. 8, the three-way linear solenoid valve comprises a spool enabling communication between a supply port and an output port/drain port. By raising an exciting current of a solenoid which drives the spool to a rated maximum value such that the drain port is opened widely, foreign matter (sludge and the like) which has accumulated around the drain port is removed by the hydraulic fluid flowing into the drain port.
Patent No. 2629512 discloses a linear solenoid valve used to control a lock-up clutch. In this prior art, control to remove foreign matter is performed only when the lock-up clutch is in a state of disengagement.